The Church has clipped the first words off the Gospel. Where it says, “Jesus took Peter, James, and John, and led them up a high mountain” the Bible says. “After six days Jesus took Peter, James and John.” We should see that St. Mark purposely linked Jesus going up on that mountain to what had happened six days earlier.
Six days earlier Jesus had revealed to the apostles that he was headed for Jerusalem where he would be mocked, scorned, and crucified. Mark is telling us that Jesus was so downcast over what he was to endure that he went up a mountain to get as close as possible to his heavenly Father. He was looking for the strength for enduring what lay ahead of him. Up there he prayed so long that the apostles fell asleep. When they awoke they were dazzled by witnessing Jesus receiving a foretaste of heaven.
The scene above them consisted in the floor of God’s heaven, like a trampoline, being stretched down beneath the feet of Jesus, so that his body was transformed by heavenly brightness.
With the Jews believing that Elijah had been taken up to heaven in a chariot, and with the grave of Moses in Bethpeor having been taken up there; those two were the only ones up there. They were free to stroll over to chat with Jesus in his foretaste of heaven.
To put the cap on the encouragement the Father was giving to Jesus, he spoke out, owning Jesus to be his beloved Son.